This invention relates to the preparation of macrocyclic oligomer compositions, and more particularly to a preparative method involving reductive dehalogenation or a process analogous thereto.
Macrocyclic polyimide oligomers are described in U.S. Pat. No. 5,357,029. The method for their preparation described therein involves the reaction of at least one diamine with at least one aromatic tetracarboxylic acid or dianhydride thereof under specified conditions, including high dilution. Other methods requiring high dilution have also been described. The macrocyclic oligomers thus obtained are capable of ring-opening polymerization to form high molecular weight linear polyimides.
The preparation of various types of macrocyclic oligomers, similarly capable of ring-opening polymerization, is described in European patent application 317,226. This method involves a reductive dehalogenation procedure employing a metal such as nickel, typically in the zerovalent state as the result of reduction by a more electropositive metal such as zinc and in the form of a complex with a triarylphosphine. It also requires high dilution, the procedure including a step of dropwise addition of a solution of a dihalo compound to a suspension of the catalyst. A typical solvent employed is dimethylacetamide. The products are described as pure macrocyclic oligomers containing one or two structural units, as shown by various analytical methods including their very high and relatively narrow melting point ranges.
From a commercial standpoint, it would be desirable to minimize the need for solvents in the production of macrocyclic oligomers capable of polymerization. It would also be desirable to obtain as a product a mixture of macrocyclic oligomers rather than a single oligomer, since such mixtures have much lower melting points than individual molecular species and may thus be handled and polymerized in liquid form at much lower temperatures.
Accordingly, the present invention provides a method for the preparation of macrocyclic oligomers which is not dependent on dissolution of the monomeric precursor in a solvent and which produces macrocyclic oligomers in the form of a relatively low melting mixture.